external mail server help

i have a website on my ispc box that has an external mail server in our LAN on another subnet. when i try to send email from the ispc box via php script to this particular domain, it never goes anywhere. the email just disappears. i'm sure it a config problem on my part, i just don't know what. any help is greatly appreciated.

I went and double checked, and i had already done that. let me mention this. my DNS for this domain points to external IP's, and my box is natted behind a firewall using privite IP's. would this make a difference??

yep, I'm sure. i double checked once again just to be sure. under options of the main domain, and under options of the co-domain, the external mail server option is selected in the drop down. any other ideas??

72.242.169.182 is my server. i have a range that sits on my firewall. my firewall will not allow me to hit an external ip from an internal ip. this is the only domain i have on this box that has an external mailserver. all the others use the mailserver on the ipsc box. any ideas???